The present invention relates to a process for recovering or purifying a nitrile from a mixture comprising the nitrile and hydrogen cyanide and to a process for producing a nitrile, especially acrylonitrile, from a hydrocarbon and ammonia.
A nitrile such as acrylonitrile or its derivative such as methacrylonitrile, is an important industrial chemical, especially in the plastics, surface coatings, and adhesive industries. For example, acrylonitrile can be used to produce acrylic fiber, as intermediate in the synthesis of antioxidants, pharmaceuticals, dyes, and surface-active agents. It can also be used as a modifier for natural polymers or as a pesticide fumigant agent for stored grain.
The production of acrylonitrile or its derivative by the catalytic ammoxidation of a hydrocarbon such as an olefin is well known and widely used. For example, the hydrocarbon used for producing acrylonitrile is propylene or propane. In this process, the hydrocarbon, ammonia and air are reacted over a catalyst at an elevated temperature, producing a vaporous mixture of acrylonitrile, acetonitrile, and hydrogen cyanide, along with water and other side-reaction products. The hot vapor is then cooled and quenched with sulfuric acid to remove unreacted ammonia. The vapor stream is then sent to a recovery system. It is first absorbed in water to create an aqueous stream containing the products of the reaction: acrylonitrile, acetonitrile, and hydrogen cyanide. The aqueous stream is then treated in a series of distillation columns to recover and purify these products. After the acetonitrile is removed for recovery, the hydrogen cyanide is stripped from the acrylonitrile stream, sent to a purification column, and chilled for storage.
Acetic acid is generally introduced during the separation of hydrogen cyanide from acrylonitrile. However, the use of acetic acid in the process has some disadvantages. Some of the acetic acid remains with the acrylonitrile stream, causing corrosion of metal equipment, and potentially remaining as an impurity in the acrylonitrile. In addition, it reacts with residual ammonia to form ammonium acetate, which is then carried by recycle streams back to the first part of the recovery and purification systems where it tends to decompose or release harmful ammonia to the process.
Stronger mineral acids such as phosphoric and sulfuric acids are also often used in processes for the direct production of hydrogen cyanide from methane or methanol as the carbon source. Since these processes produce few or little recoverable byproducts, the mineral acids do not affect the system adversely. However, mineral acids cannot be used in processes where acrylonitrile is the main product because they can react with acrylonitrile, especially under anhydrous conditions such as are found in acrylonitrile purification processes.
Therefore, there is an increasing need to develop a process for the purification of a nitrile such as, for example, acrylonitrile.
This invention comprises (1) contacting a mixture, which comprises a nitrile and hydrogen cyanide, with an acid having a pKa xe2x89xa64.4 or a precursor of the acid to produce an acid-treated mixture; and (2) recovering the nitrile from the acid-treated mixture wherein the acid is not glycolic acid.
According to the invention, the product mixture can be obtained from any source as long as the product mixture comprises a nitrile and hydrogen cyanide. Presently it is preferred that the product mixture be produced by a process which comprises contacting a hydrocarbon such as, for example, an olefm with ammonia and air to produce a product mixture comprising a nitrile and hydrogen cyanide.
The product mixture, which comprises a nitrile and hydrogen cyanide, is generally contacted with an acid, or a compound that produces the acid in-situ, to produce an acid-treated product. According to the invention, the product mixture can be contacted with the acid during the recovery or purification of the nitrile. The acid preferably has a pKa of xe2x89xa64.4, more preferably xe2x89xa64.3, and most preferably xe2x89xa64.2. Generally, the lower end of the pKa is about 1.9. Examples of suitable acids include, but are not limited to, succinic acid, lactic acid, formic acid, glyceric acid, citric acid, fumaric acid, citraconic acid, maleic acid, sulfamic acid, esters of these acids, and combinations of two or more thereof.
The amount of acid or acid precursor used can be any quantity so long as the amount can substantially prevent the polymerization of the product mixture (or a component thereof) thereby facilitating the recovery or purification of the nitrile such as acrylonitrile. Generally, the molar ratio of acid to hydrogen cyanide present in the product mixture can be in the range of from about 0.001:1 to about 1000:1. Alternatively, the amount of acid or acid precursor is the amount that can lower the pH of the product mixture to about 4.2 or lower. The contacting can generally be carried out at a temperature in the range of from about 25xc2x0 C. to about 130xc2x0 C., preferably about 30xc2x0 C. to about 100xc2x0 C., under a pressure that can accommodate the temperature range, and for a time sufficient to separate, recover, or purify the nitrile, generally about 10 seconds to about 2 hours.
According to the invention, the term xe2x80x9cnitrilexe2x80x9d refers to a compound having the formula of RCN in which R is a hydrocarbyl radical having 1 to about 10 carbon atoms per radical. The presently preferred nitrites are acrylonitrile, methacrylonitrile, or combinations thereof. The hydrocarbon can be ethylene, propylene, isobutylene, butene, pentene, propane, or combinations of two or more thereof. The presently preferred hydrocarbon is propylene because acrylonitrile can be produced therefrom.
The contacting of hydrocarbon with ammonia and air is generally carried out in the gas phase in a suitable vessel such as, for example, a fluidized bed reactor having an air compressor and a quench column. A hydrocarbon such as, for example, propylene and ammonia can be vaporized and introduced into the vessel or reactor. The molar ratio of hydrocarbon to ammonia can be any ratio so long as a nitrile can be produced. Generally, the molar ratio can be in the range of from about 0.1:1 to about 10:1, preferably about 0.2:1 to about 5:1, and most preferably about 0.5:1 to about 2:1. The contacting can be carried out under any suitable condition such as a temperature in the range of from about 250 to about 600, preferably about 300 to about 550, and most preferably about 350 to about 500xc2x0 C., under a pressure that can accommodate the temperature range, and for a time sufficient to produce a nitrile, generally about 10 seconds to about 2 hours.
The contacting of hydrocarbon with ammonia and air can also be carried out in the presence of an ammoxidation catalyst, as disclosed in U.S. Pat. Nos. 3,936,360 and 4,981,670, the disclosures of which are incorporated herein by reference. Because an amnuoxidation catalyst is well known to one skilled in the art, the disclosure of which is omitted herein for the interest of brevity. The contacting produces a product mixture comprising a nitrile and hydrogen cyanide, generally in gas or vapor phase.
Generally, a product mixture produced any source is cooled to a temperature in the range of from about 200 to about 270xc2x0 C. to produce a cooled product mixture. The cooled product mixture is then quenched, with a quenching solution that comprises water and a recycled stream as defined in U.S. Pat. No. 3,936,360, to about 30 to about 90xc2x0 C. to produce a quenched mixture. The quenched mixture is then contacted with sulfuic acid at about 70 to about 90xc2x0 C. The amount of sulfuric acid can be any amount as long as it is sufficient to react any excess or ammonia to produce a quenched product.
The spent quenching solution can be generally further treated such that high-boiling materials, primarily catalyst fines, tars and other organic materials are concentrated, cooled, and routed to waste treatment along with the water generated by the contacting of olefm and ammonia.
Generally, the product mixture vapor can be routed to an absorber in which the organic materials are absorbed in chilled water. Nitrile and hydrogen cyanide, in the aqueous stream from the absorber containing the dissolved organic materials, are separated from the bulk of the water. Generally the separation can be carried out by azeotropic-extractive distillation, using water as the solvent. If the olefin is propylene, acetonitrile is generally also present in the product mixture and can be separated from acrylonitrile and hydrogen cyanide. The acrylonitrile and hydrogen cyanide are removed via the overhead stream and can be further purified. Acetonitrile and other organic materials are separated from the water in the bottom stream, which contains water and acetonitrile. The water can be recycled to serve as the solvent in both the absorber and the recovery of nitrile.
The product mixture can be further purified by any means known to one skilled in the art such as that disclosed in U.S. Pat. No. 3,936,360. Because the purification is well known, the description of which is omitted herein.